A 118-year-old cylinder that has been the international prototype for the metric mass, and kept under lock and key outside Paris, is mysteriously losing its weight - if ever so slightly.

Physicist Richard Davis of the International Bureau of Weights and Measures in Sévres, south-west of Paris, says the reference kilo appears to have lost 50 micrograms - roughly equivalent to the weight of a fingerprint - compared with the average of dozens of copies.

"The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart," said Mr Davis yesterday in a telephone interview. "We don't really have a good hypothesis for it."

Only the one in Sévres really counts. It is kept in a triple-locked safe at a chateau and only rarely sees the light of day - mostly for comparison with other cylinders shipped in periodically from around the world.

For scientists who rely on the official kilogram for minute measurements every day, the inconstant metric constant is a nuisance - threatening calculation of things such as electricity generation.